For example, in an overlock sewing machine, while cutting a cloth edge using an upper knife moving vertically in interlocking with a sewing machine main shaft, the cloth edge is sewn.
And, there is known an overlock sewing machine which, when the upper knife is not used, releases its interlock with the sewing machine main shaft and stops it below a needle plate to set it into its retreat state where it is retreated from its cloth edge cutting position, thereby facilitating cloth handling on the needle plate (see, for example, Japanese Patent Publication No. 4383842).
Also, in another sewing machine 200 shown in FIG. 14, when an upper knife 201 is released from its interlock with a sewing machine main shaft, it stops at its projection position on a needle plate 202, thereby enabling the upper knife 201 to be used as a cloth guide.
In the sewing machine of JP Publication No. 4383842, when the upper knife is set in the retreated state, cloth handling on the needle plate is facilitated. However, for a user not familiar with a sewing machine, the freedom of use is too high, thereby raising a fear that the finished state can be unstable.
Also, in the sewing machine 200 of FIG. 14, since the upper knife 201 remains projected from above the needle plate 202, in a sewing operation not requiring a cloth guide, the upper knife 201 provides an obstacle.